Background: Mycobacterial burden is low in extrapulmonary specimens, making diagnosis and treatment difficult. Xpert MTB/RIF is a real-time PCR assay for the detection of Mycobacterium tuberculosis and rifampin resistance. This study evaluated the performance of the Xpert MTB/RIF assay in extrapulmonary specimens.
Methods: Acid-Fast Bacilli (AFB) smear, culture, and Xpert MTB/RIF were performed on extrapulmonary specimens. Mycobacterial culture was performed on BACTEC MGIT liquid for 6 weeks and 2% Ogawa medium for 8 weeks. Overall sensitivity and specificity of Xpert MTB/RIF was estimated using culture as a gold standard. Xpert MTB/RIF sensitivity and cycle-threshold (Ct) values according to AFB smear grade were evaluated. The sensitivity, specificity, and concordance of rifampin resistance compared to the phenotypic drug sensitivity test were evaluated.
Results: A total of 1,289 specimens were included in the study. The overall sensitivity and specificity of the Xpert MTB/RIF assay were 59.4% (41/69, 95% CI 46.9 - 70.9%) and 99.3% (1,212/1,220, 95% CI 98.7 - 99.7), respectively. Positive predictive value of Xpert MTB/RIF was 83.7% (41/49, 95% CI 69.8 - 92.2) and negative predictive value was 97.7% (1,212/1,240, 95% CI 96.7 - 98.5%). Xpert MTB/RIF assay sensitivity significantly increased with increases in AFB smear grade (p < 0.001). AFB smear grades and Xpert MTB/RIF Ct values were negatively correlated. Rifampin resistance results of Xpert MTB/RIF and culture showed a concordance rate of 97.2%.
Conclusions: The Xpert MTB/RIF assay could be used to replace the AFB smear for the diagnosis of extrapulmonary tuberculosis, and has high specificity for the detection of rifampin resistance.